I feel that any number(within reason) can be successful. However I feel that you need to get the right mix to your clan. Like some clans have have 1 or 2 leaders and they are the "big boss". But other clans don't have one main leader and maybe the head of events, head of pking, head of recruitment etc. My opinion is that 2 is the right number. My reason for this is that if one leader has an idea about something and t's not very good at all, then the other leader can knock some sene in to the other. Another reason is that if one leader decides to leave the clan then there is a leader left to run things, then think about who would be the best to be their next partner.